1. Field of the Invention
This invention relates to pilot signal transmission systems, and more particularly to those systems for synchronizing a pilot signal for a multilevel amplitude modulated signal converted from a digital signal.
2. Description of the Prior Art
In data transmission, synchronization is necessary for reproducing transmitted signals. To this end, there has heretofore been adopted a system for synchronizing a clock signal on the receiving side of the transmission line with the symbol rate on the transmitting side or, in the carrier system, a system for synchronization between transmission and reception for synchronous detection. Where the number of the levels is smaller than four, a synchronizing signal can be extracted from the multilevel signal itself on the receiving side but jitter increases with an increase in the number of the levels to make reproduction of the synchronizing signal difficult. To avoid this, a method is generally adopted such that the transmitted signal is transmitted with the synchronizing pilot signal being coupled therewith on the transmitting side and that, on the receiving side, the synchronizing pilot signal is extracted for synchronous reproduction. It is easy to extract the pilot signal because its frequency is selected outside of the transmission signal band, but this technique requires extra band capability and decreases the efficiency of the transmission band. Accordingly, it is desirable to effect signal transmission with a pilot signal frequency being selected within the transmission band. However, there are some occasions when the interference between the transmitted signal and the pilot signal or phase jitter increases. Further, it is necessary to eliminate the pilot signal at the time of multilevel decoding on the receiving side; otherwise, the pilot signal would exert an influence on the transmitted signal level. To perform this operation, where a sinusoidal pilot signal is used, the pilot signal is generally removed by adding thereto a sine wave which is equal in frequency and amplitude, but opposite in phase thereto.
For example, FIG. 1 shows the frequency relationship between a digital signal spectrum S and a sinusoidal pilot signal P. The frequency of the pilot signal P is selected to be one-half of the symbol rate of the digital signal and, as depicted in FIG. 2, the digital signal is converted by a binary-multilevel converter 1 into a ternary, quaternary or like multilevel signal and is superimposed on the pilot signal P by a superimposing circuit 2 and transmitted from a transmitting station to a receiving station through a transmission line 3. In the receiving station, the pilot signal P is extracted by a pilot signal extracting circuit 4 for synchronous reproduction and, in a cancelling circuit 5, the pilot signal contained in the received signal is cancelled. Then, the received signal is subjected to multilevel decoding and converted by a multilevel decoding circuit 6 into a digital signal to provide an output signal at an output terminal OUT. It is necessary to dispose an equalizing and shaping filter for providing an rectangular waveform free from intersymbol interference at an appropriate position between the binary-multilevel converter 1 of the sending station and the multilevel decoding circuit 6 of the receiving station but this circuit is omitted in FIG. 2. Further, a clock circuit 7 is controlled with the output of the pilot extracting circuit 4 for synchronous reproduction. In the multilevel decoding circuit 6, the multilevel decoding is effected with the output of the clock circuit 7 and the decoding is usually achieved at the time the pilot signal is of maximum positive and negative values. Alternatively, sampling may be effected at a desired certain level of the pilot signal.
With conventional systems, where a steady phase error, phase jitter or amplitude error is caused by the incomplete operation of the pilot extracting circuit 4 to provide a digital signal by cancelling the sinusoidal pilot signal with a sine wave (as described above), the pilot signal cannot be completely removed and this introduces an error in the multilevel decoding. The incomplete operation of the pilot sampling circuit 4 is unavoidable because of its phase characteristic due to a narrow band filter provided for sampling the pilot signal from the signal spectrum and the transmitted signal serving as a source of jitter of the pilot signal. Accordingly, it is difficult to set the amplitude and phase of the pilot signal which enables the pilot signal to be completely cancelled, so that it is impossible to prevent errors.
In the transmission line of the conventional pilot synchronizing system, as shown in FIG. 3, a digital input signal is converted by a binary-multilevel converter 11 into a multilevel signal, which is rolled off by a band width control filter 12. A sine wave, which has a frequency one-half of a clock signal produced by supplying a filter 14 with a clock signal whose frequency is reduced to one-half by a clock circuit 13, is coupled with the output of the band width control filter 12 as a pilot signal. The clock circuit 13 also provides a timing signal for the binary-multilevel conversion based on the transmitted clock signal.
Further, in a modulator 15, the signal coupled with the pilot signal is modulated with a carrier derived from an oscillator 16 and is subjected to band width control by a band filter 17. A sine wave of the carrier frequency is coupled with the output of the band filter 17 as a pilot signal.
The composite signal thus obtained is transmitted from a transmission section to a receiving section through a transmission line 3' (for example, a conventional FDM line). In the receiving section, the received signal is applied through a receiving band filter 18 to a demodulator 19, to be demodulated with a carrier reproduced by a carrier reproducing circuit 20. The demodulated signal is then divided into first and second signals: the first signal is applied to a sampling timing signal reproducing circuit 21 to provide a synchronized sampling pulse, which is applied to a pilot eliminating circuit 22 to eliminate the pilot signal contained in the second signal fed thereto. The signal having removed therefrom the pilot signal is converted by a multilevel decoder 23 into a binary signal, thus completing transmission of one section of data.
In FIG. 3, reference numeral 24 indicates a circuit which provides a decoding timing signal for the multilevel decoder 23 based on the output of the sampling timing signal reproducing circuit 21.
In the coupling of the multilevel signal with the pilot signal, it is absolutely necessary to establish the phase relation therebetween phase synchronization on the receiving side and, further, for establishment of a certain phase relation therebetween is convenient for various signal processing. In regard to establishing the phase relationship, the conventional system shown in FIG. 3 presents problems such as inaccurate phase relationship between the multilevel signal and the pilot signal due to phase variations of the filters (12, 14 and 17), deterioration of the analog signal resulting from coupling of the analog signal with the pilot signal by means of the analog circuit and complexity in the circuit construction for improvements therefor.